The invention relates to a device and method for detecting noise in an electronic device, especially noise in impulse form, in a signal with a bandwidth-limited useful component, and a device and method for suppressing such noise.
Probably the most important example of this type of bandwidth-limited signal is an audio signal. The useful component of such an audio signal lies within the audible frequency range of human hearing, or approximately 40 Hz to 16,000 Hz. However, noise impulses that propagate through a line along with the useful component may have frequency components that extend beyond the audible frequency range.
Noise impulses in such an audio signal may occur for a number of reasons. For example, electrical equipment in the vicinity of a radio receiver may become noticeable as noise, such as ignition pulses from a vehicle engine. In addition, processing of the audio signal itself (e.g., from clipping of a digital signal or overflow in a digital demodulator) may result in impulse noise.
To efficiently implement noise suppression, it is first necessary to reliably detect the noise. A variety of techniques have been proposed for detecting and eliminating noise components in a signal.
U.S. Pat. No. 5,479,440 discloses a technique that is based on conversion of time data from the signal to be cleared of noise to the frequency domain, the determination of a threshold below which all frequency components are canceled, and the back-conversion of the spectrum obtained to the time domain. This technique requires a double fast Fourier transform (FFT) and is thus complex and expensive, and accordingly relatively poorly suited for use in devices intended for a broad range of consumers.
U.S. Pat. No. 4,622,520 discloses a method for eliminating noise from an audio signal modulated from a high-frequency carrier. In this method, the strength of the signal component is compared with that of the carrier, and signal components that are stronger than the carrier are detected as noise and suppressed. However, this technique cannot be applied to signals in the baseband since no carrier is available there as a reference for comparison. In addition, the application of this technique to digital signals is problematic due to the required high sampling rates.
U.S. Pat. No. 4,143,333 proposes detecting noise impulses in an audio signal using a differentiating filter. For the duration of the noise the signal is blanked out. This type of noise detection directly from the audio signal is error-prone because it must first be ensured that a loud, high tone in the audio signal is not erroneously detected as impulse noise. In addition, there is the danger that noise will go undetected and thus unattenuated if the noise detection threshold is too high.
Therefore, there is a need for a technique of detecting and suppressing noise in a bandwidth-limited signal that is relatively simple and inexpensive to implement, yet provides reliable detection and suppression.